Electrical resistors and materials therefor



United States Patent 2,992,918 ELECTRICAL RESISTORS AND MATERIALS THEREFOR Bjiim Edwin, Hallstahammar, and Giista Erik Hildebrand, Stockholm, Sweden, assignors to Akliebolaget Kanthal, Hallstahammar, Sweden No Drawing. Filed July 6, 1960, Ser. No. 41,002 Claims priority, application Sweden Nov. 12, 1953 6 Claims. (Cl. 75-171) This invention relates to electrical resistance elements and, more particularly, to electrical resistance elements formed from material adapted to be drawn in the form of extremely fine wires for use in the manufacture of wire wound precision resistors. This application is a continuation-in-part application based on our copending application Serial No. 651,595, filed on April 9, 1957, now abandoned, which application is in turn a continuationin-part of our then copending application Serial No. 401,032, filed on December 29, 1953, now abandoned.

In the manufacture of wire wound precision resistors there are employed resistance wires commonly having diameters in the range of .0005 to .0031. It will be evident that certain physical and electrical characteristics of such wires are of extreme importance. The tensile strength of wires of these minute sizes is an essential consideration in the handling of the wire in the manufacture of wire wound resistors. It is necessary that the wires have a substantially constant resistance over a wide temperature range in view of the fact that the precision resistances are operated under conditions of widely varying temperatures, i.e., temperatures ranging from sub-zero temperatures such as would be experienced in arctic regions to temperaturesas high as, for example, 150 C. to 200 C. as may be encountered in the vicinity of heat engines or other special conditions of application. The wires must have relatively high specific resistance in order that a desired resistance may be obtained Without requiring a great length of wire or without requiring wires of even smaller diameters than those noted above.

force exhibited by the material. Itwill be evident that in instrument circuits a thermo-electromotive force dc veloped within a precision wire wound resistance will further consideration involves the thermo-electromotive Patented July 18, 1961 falls between -1.5 and +1.1.

The alloy according to the invention is austenitic and thus non-magnetic.

In the alloy there may, of course, also be included other elements than those above mentioned and which are suitable from the point of view of manufacture, provided that the quantities thereof are so small, that the temperature coefiicient of the resistance of the alloy does not become higher than 55.10 ohm/ohm/ C. or the specific resistance less than 1.2 ohm mmF/m. Said elements may consist of such deoxidizing and degasifying agents as calcium, magnesium, cerium, zirconium and boron, and further metals for the stabilization of carbon, such as titanium, niobium, tantalum and vanadium. Under the conditions stated, further such elements may be included, which generally are present in nickel, such as form very hard particles. In case it is desirable to draw fine wires from the respective alloy it is self-evident that the wire could not be drawn down to a diameter approaching the size of said intermetallic compound particles. A closer examination of said particles has shown that their size and distribution is more advantageous in the silicon containing alloys than in the aluminium containing alloys.

'Based on this conception we have succeeded in drawing said silicon-containing alloy down into very fine Wires, particularly within the above mentioned range of 2.6 to 6% by weight silicon, the desirable diameters of .0005" to .0031" being then obtainable.

The presence of aluminium is further undesirable for the reason that it always exists to some degree in the form of aluminium oxide which is a highly abrasive material. Resistance wire is manufactured by drawing the wire through dies in order to reduce the diameter of the wire to diameters ranging from .0005" to .0031 as noted above. It will be evident that a slight amount of wear in a die will give rise to a substantially great peris possible to obtaina considerably improved combination of properties in certain alloy combinations in the system of nickel-chromium-silicon-manganese in respect of the temperature coefficient of resistance, the specific resistance, the thermo-electromotive force versus copper and the properties of mechanical strength in the tempera.

ture range of to +250 C. Said alloy consists to its major portion of nickel and the remainder contains from 10 to 30% of weight of chromium, 2.6 to 6% silicon and 0.4 to 6% manganese. Higher contents of silicon and manganese are undesirable for the reason that both the hot and the cold workability of the alloy is detrichromium the B value should fall between 2.1 and 4.7. M

centage change in wire diameter and thus when a length of wire is drawn which carries abrasive materials the efiect will be a wearing away of the die resulting in an increase in wire diameter from one end to the other end thereof with an accompanying non-uniformity of resistance per unit length of the wire. Precision wound resistances carry substantial lengths of wire and a substantial number of resistances are wound from an uninterrupted length of wire which is cut as the individual resistances are wound. Precision manufacture of the resistances quite obviously requires extreme uniformity in wire diameter. Thus, when aluminium oxide is present and die life is greatly reduced thereby, drawing costs are increased and the accuracy of the diameters of the drawn wire is decreased. Thus, from this manufacturing standpoint, silicon is undesirably replaced by aluminium,

A further disadvantage found when aluminium is present is the increased difliculty in soldering. It will be evident that electrical resistance wires must frequently be soldered and any increased difliculty in soldering is undesirable. Furthermore, inview of the extremely minute wire sizes involved, the wetting of the resistance wire by the solder becomes most diincult and thus the presence of oxide increasing the soldering difiiculty is even more undesirable in small diameter wires than it would be in wires of larger diameters.

It has been suggested that the manganese may be re.- placed by copper, however, the presence of copper serves to reduce the tensile strength of the material. It will be evident that the difiiculties involved in handling the extremely fine diameter wires noted above are sufficiently great when the wires possess a maximum tensile strength. Thus, any reduction in this tensile strength is highly undesirable. Therefore, from a mechanical standpoint, the manganese is undesirably replaced by copper.

The electrical and mechanical properties which are characteristic features of the present resistor are not fully inherent in the composition of the alloy but are ob tainable only by a suitable heat treatment after the wires have been drawn to their final diameter. This treatment is carried out in two steps, i.e. first an annealing of short duration at a temperature of from 850 to 950 C. and then an age annealing for /z to 24 hours at a temperature of from 425 to 525 C. The preferred values can easily be determined by a few trials.

The duration of the first annealing is rather short, advantageously from one second to 100 seconds, but may be extended to a longer period without materially changing the final properties. This first annealing has for its purpose to dissolve silicon and to resolve the internal stresses in the material.

To illustrate the importance of the heat treatment on the properties of the resistance wires and particularly on the temperature coefficient of the resistance the following measurements may be cited, made on wires drawn from two melts.

Each melt had a weight of about 200 kilograms or more and after melting and pouring it to form ingots, these were rolled down to wires of a diameter of A" and finally drawn to a diameter of .002".

The two melts were chemically analysed and their compositions were found to be:

Si Mn Cr Balance Charge 1 3. 9 3. 6 17.6 Ni Charge 2... 4. 1 3. 4 17.1 Ni

' having a non-oxidizing atmosphere heated to 920 C., and,

secondly, after the wires had been aged. The ageing was carried out in a convection furnace in which the wires wound on spools were placed in a container having a nonoxidizing atmosphere of cracked ammonia and heated to a temperature of 475 C. for a period of 10' hours and then removed. The resulting temperature coefficients were as follows:

Charge 1 Charge 2 Temperature ranges Drawn Drawn and Aged and Aged annealed annealed -50 to +20 C 60.0Xl' 6.0Xl0" 54.5Xl0' -1.7X' +20 to +150 C 56.1)(10 -4.0 l0 53.9)(10' -9.9 10

The results indicate a very pronounced reduction of the temperature coefficient of the wires after ageing as compared with the values before ageing.

The following examples of compositions of alloys' according to the present invention are set forth toillustrate the different combinations of properties after the wires have been drawn to a diameter of .002 and been subjected to heat treatment, including annealing for 15 seconds at 860 C. and age'annealing for 3.5 hours at about 500" C.

Example 1.Chromiurn 20% of weight, silicon 4.1%, manganese 1.9% and balance nickel. This alloy will have the following properties, i.e., a temperature coeflicientof 20 10 ohm/ohm/ C., a thermo-electromotive force versus copper of 0.9 microvolt/ C., a specific resistance of 1.30 ohm mmP/m. and a rupture strength of 115 kilograms/mmfl.

Example 2.--Chromium 20% of weight, silicon 4.9%, manganese 2.7% and balance nickel. This alloy will have the following properties, i.e., a temperature coefficient of 18 10- ohm/ohm/ C., a thermo-electromotive force versus copper of 0.1 microvolt/ C., a specific resistance of 1.30 ohm mmF/m. and a rupture strength of 135 kg./mm.

Example 3 .-Chromium 14.5% of weight, silicon 3.9%, manganese 3.2% and balance nickel. This alloy will have the following properties, i.e., a temperature coefficient of 1X10" ohm/ohm/ C., a thermo-electromotive force versus copper of 3.5 microvolts/ C., a specific resistance of 1.29 ohm mmF/m. and a rupture strength of 115 kg./mm.

Example 4.Chromium 19.9% of weight, silicon 3.0%, manganese 3.6% and balance nickel. This alloy will have the following properties, i.e., a temperature coefl'icient of x10- ohm/ohm/ C., a thermo-electromotive force versus copper of l microvolt/ C., a specific resistance of 1.23 ohm mmF/m. and a rupture strength of 95 kg./m=m.

Example 5.--Chromium 15% of weight, silicon 2.6%., manganese 1.2% and balance nickel. This alloy will have the following properties, i.e., a temperature coefficient of 3X10 ohm/ohm/ C., a thermo-electromotive force versus copper of 4 microvolts/ C., a specific resistance of 1.21 ohm mnrfi/m. and a rupture strength of 95 kg./mrn-.

Example 6 .-Chromium 17.5% by weight, silicon 4.0% manganese 3.5% and balance nickel. This alloy will have the following properties, i.e. a temperature coeflicient of 5X10 ohm/0hm/ C., a thermo-electromotive force versus copper of 1.5 microvolts/ C., a specific resistance of 1.33 ohm mmR/m. and a rupture strength of 95 leg/mm From the above description of the present invention and the specific embodiments thereof, it is believed apparent that the same may be widely varied without 688611, tial departure therefrom and all such are contemplated as may fall within the scope of the following claims.

What we claim is:

1. An electric resistor comprising wires of a diameter of from .0005 to .0031" and drawn from an alloy" consisting of 10 to 30% by'weight chromium, 2.6 to 6% by weight silicon, .4 to 6% by weight manganese and the balance nickel and annealed for a length of time from 1 second upwards at a temperature of 850 to 950 C. and subsequently age annealed for a length of time from /2 hour up to 24 hours at a temperature of 425 to 525 C., said wire having an electric resistance higher than 1.20 ohm mm. /m. and a temperature coefiicient of resistance below 55x10 ohm/ohm/ C.

2. The electric resistor of claim 1, in which the percentages of weight for silicon and manganese are selected according to the equation Si percent=.4 Mn percent-H3,

where B has a value between .6 and 4.8, the value of B being based on the chromium content in accordance with the equation B.=.l.8 Cr percent-i-C with the values of C falling between 1.5 and 1.1.

' 3. The electric resistor of claim 1, in which the per-i centa'gesof weight for silicon are 3.5 .to 5 and for'im ane ganese 3 to 5. l i

4. The electric resistor of claim 3, in which the percentages of weight for chromium are 15 to 25.

5. The electric resistor of claim 1, in which the percentages of weight for chromium are 15 to 20, for silicon 3.5 to 4.5 and for manganese 3 to 4.5.

6. An electrical resistance element consisting essentially of a wire of a diameter of from .0005" to .0031" drawn from an alloy consisting essentially of 10% to 30% by weight chromium, 2.6% to 6% by weight silicon, 0.4% to 6% by weight manganese and the balance nickel and heat treated, said wire having an electric resistance higher than 1.20 ohm mmP/m. and a maximum temperature coeflicient of resistance of 55 10- ohm/ohm/ C.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ELECTRIC RESISTOR COMPRISING WIRES OF A DIAMETER OF FROM .0005 TO .0031" AND DRAWN FROM AN ALLOY CONSISTING OF 10 TO 30% BY WEIGHT CHROMIUM, 2.6 TO 6% BY WEIGHT SILICON, .4 TO 6% BY WEIGHT MANGANESE AND THE BALANCE NICKEL AND ANNEALED FOR A LENGTH OF TIME FROM 1 SECOND UPWARDS AT A TEMPERATURE OF 850 TO 950*C. AND SUBSEQUENTLY AGE ANNEALED FOR A LENGTH OF TIME FROM 1/2 HOUR UP TO 24 HOURS AT A TEMPERATURE OF 425 TO 525*C., SAID WIRE HAVING AN ELECTRIC RESISTANCE HIGHER THAN 1.20 OHM MM.2/M. AND A TEMPERATURE COEFFICIENT OF RESISTANCE BELOW 55 X 10**6 OHM/OHM/*C. 